Plasma processing systems are commonly used for modifying the surface properties of substrates in various industrial applications. For example, plasma processing systems are routinely used to plasma treat the surfaces of integrated circuits, electronic packages, and printed circuit boards in semiconductor applications, solar panels, hydrogen fuel cell components, automotive components, and rectangular glass substrates used in flat panel displays. Plasma processing systems are also used in medical applications to modify the surface properties of devices, such as stents and implants, inserted into the human body. Plasma processing systems that rely on conventional parallel-plate type electrodes may experience process non-uniformities across the surface of relatively large substrates positioned in a processing region defined between the electrodes for processing.
When radio frequency power is supplied to the electrodes, equipotential field lines are induced across the surface of the substrate. During plasma processing, positive ions from the plasma in the processing region accelerate across the equipotential field lines to impinge on the surface of the substrate. The plasma is typically distributed over the entire evacuated volume of a processing chamber enclosing the electrodes with the highest plasma density observed between the electrodes. The uniformity of the plasma density in the processing region between the electrodes is influenced by external field effects factors, such as grounded chamber sidewalls, that alter the equipotential electric field lines between the electrodes and thereby modify the distribution of the constituent charged components of the plasma. The non-uniformity may be particularly significant at the peripheral edges of the processing region.
One conventional method of reducing external field effects is to make the processing chamber larger so that the grounded sidewalls are more distant from the electrodes. Among other disadvantages, this increases the chamber volume and the footprint of the processing system. The increase in chamber volume increases the time to evacuate the processing chamber and the time to bleed or vent the processing chamber to atmospheric pressure to insert unprocessed substrates or remove processed substrates. In particular, these are especially undesirable effects that significantly reduce throughput in in-line plasma processing systems intended to serially plasma process large quantities of substrates, which requires periodic evacuation and venting to exchange substrates after each processing cycle.
Another disadvantage of conventional plasma processing systems is that plasma is inadvertently generated in evacuated regions inside the processing chamber peripheral to the processing region between the electrodes. The generation of plasma in these regions renders the plasma process difficult to control and may damage components positioned within these regions. This unconfined plasma may also change the location of power absorbed by the plasma within the plasma processing chamber, thereby making it difficult to control the delivery of power to the electrodes to achieve consistent and reproducible processing.
Conventional approaches for confining the plasma generally include the use of repulsive fields, either electric or magnetic in nature. One specific conventional approach is to position confinement rings about the outer periphery of the parallel-plate type electrodes. The confinement rings, which are formed from an electrical insulator, charge to a potential comparable to that of the plasma, which generates a repulsive electric field that laterally confines the plasma. Nonetheless, the electrodes and confinement rings are still positioned inside of, and surrounded by, a considerably larger vacuum chamber that must be evacuated and in which a plasma discharge may still exist. The confinement rings are arranged with gaps so that the processing region defined between the electrodes is adequately evacuated.
It would therefore be desirable to provide a plasma processing system that overcomes these and other deficiencies of conventional plasma processing systems, as described herein.